It depends... Just because the birds are older doesn't automatically mean they can be fed a reduced CP diet, you have to take into consideration their rate of lay. If they are well off their peak rate of lay then you can probably run a 16% CP/0.3% available P diet pretty easily. If the weather's getting cold, though, you should consider the caloric value of the feed, too. Ca should remain the same, about 3.6%-3.8% of the diet (as Ca) which translates out to around 9%-10% limestone or oyster shell (both are 38 wt% Ca).
I'd assume, given the photoperiod, that they are probably about shut down, so you should be fine with a reduced CP diet, as long as you don't get too low on the lysine/methionine/threonine levels. Lysine especially, it's the first limiting amino acid for chickens, and shouldn't fall below about 1% or so of the diet.
Older hens tend to throw larger eggs, and since a hen can normally only put on a fixed amount of shell, that means that the shells tend to get thinner with increased size. If you are seeing body checks or shell cracking, try dusting a little Clinoptilolite zeolite in with the feed (1%-2% tops). Clino tends to modify the uptake of Ca and P in the gut so that less is lost and you'll wind up with improved shell strength.
Hens are only about 50% digestively efficient. Especially for inorganic additives like Ca and P, particle size and molecular composition affect bioavailability and overall uptake rate, but that can be beneficially shifted to some extent by the Clino.
